When reasoning text grows during streaming, new parts can be appended
beyond endIndex. The pending check used slice(startIndex, endIndex)
which excluded these new parts — if the original part completed, the
block would close while new reasoning was still streaming.
Fix: remove the endIndex cap from slice() so all parts from startIndex
onward are checked. During non-streaming, the array is stable and
all parts are within range anyway.
ea44011d15